The present invention relates to subterranean treatments and operations, and more specifically, to additives that may be useful in preventing fluid loss in certain subterranean formations, and associated methods of use.
Providing effective fluid loss control for subterranean treatment fluids is highly desirable. “Fluid loss,” as that term is used herein, refers to the undesirable migration or loss of fluids (such as the fluid portion of a drilling mud or cement slurry) into a subterranean formation and/or a proppant pack. The term “proppant pack,” as used herein, refers to a collection of a mass of proppant particulates within a fracture or open space in a subterranean formation. These “treatment fluids” may comprise any fluids used in a subterranean application. As used herein, the term “treatment” does not imply any particular action by the fluid or any component thereof. Treatment fluids may be used in any number of subterranean operations, including drilling operations, fracturing operations, acidizing operations, gravel-packing operations, acidizing operations, well bore clean-out operations, and the like. Fluid loss may be problematic in any number of these operations. In fracturing treatments, for example, fluid loss into the formation may result in a reduction in fluid efficiency, such that the fracturing fluid cannot propagate the fracture as desired.
Fluid loss control materials are additives that lower the volume of a filtrate that passes through a filter medium. Certain particulate materials may be used as a fluid loss control materials in subterranean treatment fluids to fill the pore spaces in a formation matrix and/or proppant pack and/or to contact the surface of a formation face and/or proppant pack, thereby forming a filter cake that blocks the pore spaces in the formation or proppant pack, and prevents fluid loss therein. However, the use of certain particulate fluid loss control materials may be problematic. For instance, the sizes of the particulates may not be optimized for the pore spaces in a particular formation matrix and/or proppant pack and, as a result, may increase the risk of invasion of the particulate material into the interior of the formation matrix, which may greatly increase the difficulty of removal by subsequent remedial treatments. Additionally, once fluid loss control is no longer required, for example, after completing a treatment, remedial treatments may be required to remove the previously-placed fluid loss control materials, inter alia, so that a well may be placed into production. However, particulates that have become lodged in pore spaces and/or pore throats in the formation matrix and/or proppant pack may be difficult and/or costly to remove. Moreover, certain particulate fluid loss control materials may not be effective in low-permeability formations (e.g., formations with a permeability below about 1 milidarcy (“md”)) since the leakoff rate in those formations is not high enough to pull the particulates into the pore spaces or into contact with the surface of the formation face and/or proppant pack so as to block or seal off the pore spaces therein.
Gelled fluids and fluid loss control “pills” comprising high-molecular weight polymers and/or crosslinked polymers have also been used to improve fluid loss control. “Crosslinked polymers” are polymers wherein two or more of the polymer molecules have become “crosslinked” by interaction with a “crosslinking agent,” such as a metal ion or a borate ion. When included in a treatment fluid, these crosslinked or uncrosslinked polymeric materials may viscosify that fluid, thereby reducing the leakoff rate of the fluid into the formation and/or proppant pack. Crosslinked or uncrosslinked polymer molecules also may reduce fluid loss by filling the pore spaces of the formation matrix and/or proppant pack, thereby preventing the flow of fluid through those pore spaces.
In many subterranean operations, it is may be desirable to remove most or all of these fluid loss materials from the subterranean formation after use, among other purposes, to restore permeability of the formation for subsequent production of fluids out of the formation. Certain breakers have been used to break down polymeric fluid loss additives in subterranean formations. Where the fluid loss additive comprises a crosslinked polymer, the crosslinking interaction may be reversed (e.g., by contacting the crosslinked polymer with an acid or low-pH fluid that de-activates pH sensitive crosslinking agents) and the uncrosslinked polymeric material may be removed from the subterranean formation or permitted to leak off into the formation.
However, the use of conventional polymeric fluid loss additives also may be problematic. Specifically, it may be difficult to remove or break certain polymeric fluid loss additives to restore the formation to a high permeability. Certain polymers may require strong external breakers to break down the polymeric structure, which may be hazardous or expensive to use. In some cases, basic and/or high pH fluids may be present or introduced into the subterranean formation, for example, to displace the fluid loss additives and/or other substances in the formation. If sufficient amounts of the crosslinking agent and uncrosslinked polymeric fluid loss additive remain in the subterranean formation, the crosslinking agent may be re-activated and re-crosslink portions of the polymeric material. This may, among other things, reduce the permeability of the formation and hinder production of fluids from the formation.